Hello everyone, welcome again to my backyard. I'd like to spend just a few minutes with you at this time. Talking to you about the cranial nerves and the spinal nerves. And this will set up a tour of the cranial nerves that I'd like to show you in the laboratory with a real human brain. Well, my learning objectives for you are first, I want you to be able to discuss the surface features of each of the three subdivisions of the brain stem. And so, in order to understand how the cranial nerves fit in, you need to know something about the divisions of the brain stem. And as you become more and more familiar with both the brain stem and the cranial nerves, I really want you to accomplish this learning objective. I want you to be able to sketch the relationships of the cranial nerves, specifically nerves three through 12, to the three subdivisions of the brain stem. So that may mean that you need to find a large piece of paper. a whiteboard, a chalkboard, maybe even just some dirt and a stick where you can make some sketches. Wouldn't that be fun to draw pictures of the brain in the dirt at your feet? But whatever it takes. I would encourage you to make visible your knowledge and your understanding of human neuroanatomy. In this case, the challenge is the brain stem and knowing where. These cranial nerves three through 12 attach. Well, I want to led by giving you a table. This is from our textbook for those of you that have the textbook, if, if you don't you can follow along in the handout that I've made available to you. Or simply follow along on this chart. And what I've listed for you here in this chart, are the 12 nerves that traditionally are called cranial nerves. And I just want to briefly walk through each one. And then show you with some graphics how they relate to the brain stem itself. And then finally, we'll have a bit of a break and you'll see me next in the lab with a, ho, human brain in my hand and I'll show you these nerves as they actually present in a human brain specimen. And I'll also make use of a brain stem model that we have that will help clarify some of the nerves that are, are more difficult to retain in the autopsy specimen. Okay, so let's begin with our cranial nerves. So it's conventional that we identify cranial nerve one as a cranial nerve. it's the olfactory nerve. cranial nerve one, however, is not actually what we see on the bottom of the brain. cranial nerve one, or the axons that penetrate through. The anterior, floor of the cranium in a region called the cribriform plate, tiny axons that, run from the nasal epithelium where our sensory cells live thru the cribriform plate and they make synaptic connections in the brain and that structure in the brain is called the olfactory bulb. So when you see the olfactory bulb don't mistake it for a cranial nerve. Know that that's part of the brain, but it's the part that receives cranial nerve one. Okay, cranial nerve two is the optic nerve. So the optic nerve, very important nerve. It communicates visual signals from the retina back to the rest of the brain. And it's also a bit unusual. We call this a nerve. But, in truth, it's an outgrowth of the diencephalon. As the brain develops, something called the eye cup forms off the lateral margins of the diencephalon. And this continues to grow outward, and forms the neural elements of the retina. So, in truth, the retina and the optic nerve are an extension of the brain. So we might think that for that reason we wouldn't call this a nerve since it's not a true nerve in the conventional sense. But in keeping with our tradition in the nomenclature of these nerves in medical nerve science, we will in fact call cranial nerve two the optic nerve. Alright. Enough trick questions about cranial nerves. Cranial nerve three is the ocular motor nerve and it's a very important motor nerve. And as we see in our table here it's involved in mediating eye movements as well as the motor arm of the pupillary light relex. So we'll talk about all of this in more detail. in a little but when we get into the motor system and also when we get into the visual system we'll come back and spend quite a bit of time actually talking about nerves two and three in that context. Well nerve four is a very small nerve unusable in a few respects that I'll mention when we get there. But it's also a motor nerve and is involved in the control of eye movements. Now that brings us to the trigeminal nerve, nerve number five. The trigeminal nerve is a very large nerve, very important nerve in the cranial region. It provides for somatic sensory signals to pass into the brain stem. And it also allows for, motor signals to leave the brain stem. to innervate the muscles that are involved in chewing. So, the trigeminal nerve is involved in both somatic sensation from the face, mouth, as well as the cornea of the eye. But also in the muscles of chewing, and we call that mastication. Okay, nerve six, the abducens nerve, this is the last of the three nerves that pertain to the movements of the eye within the orbit. So this is a motor nerve, and as its name implies, it abducts or a b ducts, moves lateral, the eye within the orbit. Okay, now we're getting into, some interesting nerves. I'm going to take nerves seven, nine, and ten together. So, nerves seven, nine, ten. We'll come back to eight in just a moment. The facial nerve, the Glossopharyngeal nerve and teh vagus nerve. So, what I'd like to highlight is that each of these nerves Is a mixture of both sensory and motor signals. So there's a sensory component to each of these and a motor component to each of these. So while on the face of it these seem like the most complicated of all the cranial nerves, and indeed they are, but they're all complicated in the same way. They all have sensory components. They all have motor components. Now this table's not exhaustive, but I'll just highlight. Some of the principle functions of each of these nerves. What I want you to take away at this point for the facial nerve is that it's involved in providing our sense of taste from the anterior part of the tongue. So that's an important sensory compoenent of the facial nerve. An important motor component is that it is involved in governing the muscles of facial expression. So next time you smile or next time you, are in a social situation and you're observing just the full range of expressions that we humans express remember that it's the facial nerve that's motivating the contraction of those facial muscles. So facial nerve, muscles of facial expression. And taste from the anterior two thirds of the tounge. Nerves nine and ten, again, complicated nerves. Complicated in the same way. Nerve nine is the Glossopharyngeal nerve, that tells you something what is does. Glosso is a word the implies the tongues and pharyngeal implies the pharynx. So, it's a nerve that is related to the tongue and the pharynx. It conveys sensory signals about taste from the tongue along with nerve seven and it's involved with providing motor output to muscles around the region of the larynx and the pharynx. And that's what we find here in the our table here to the right. one additional function that's worth highlighting for the glossopharyngeal nerves, is that it also provides important sensory signals, that are important to cardiovascular control. Specifically, signals derived from the carotid bodies, as well as the aortic arch. Okay. And, nerve ten, the vagus nerve. Like glossopharyngeal, it's also complicated. It conveys sensory signals from the viscera. and it conveys a special sensory signal about taste. So, seven, nine, and ten are all concerned with taste. Among other kinds of sensory signals that are coming into the brain stem. And in terms of the vagus nerve. You may already know something about this nerve. This is a nerve that's involved in providing parasympathetic outflow to, viscera in the thorax. In the upper part of the abdomen. It's also involved in innervating parts of the pharnyx including the vocal cords so it's important in speech it's important in swallowing. Alright, so I'm going to jump back now pick up cranial nerve eight, cranial nerve eight is the vestibulocochiear nerve it's entirely sensory it's a special sensory. Nerve, it communicates back to the brain stem our sense of hearing from the cochlea and the temporal bone, but also a sense of balance, a sense of the signals that are detected by the vestibular labyrinth in the temporal bone, which allows us to sense the tilt of the head as well as it's rotational accelerations and linear accelerations. So these signals are very important in helping us maintain a sense of balance as well as helping us to maintain our point of fixation. Whatever it is we're looking at, even if we decide to move our head back and forth. So, will see demonstrations of that in a little bit and talk more about that in detail. Alright, so finally nerves 11 and 12 nerve 11 is the spinal accessory nerve the spinal accessory nerve is derived from the upper cervical segments or the spinal cord it's unusual nerve in that. It is formed by rootlets that come off the upper part of the cervical spinal cord, enter the[UNKNOWN] and magnum so now it's in the cranium, and the nerve loops around and then exits the cranium again after picking up a small contribution from the brain stem. And then that nerve exits out out of[UNKNOWN] and a with ahcCranial nerves nine and ten. The jugular for[INAUDIBLE]. So, from that standpoint, it's fair enough to call it a cranial nerve. Even though most of its origin is actually below the cranium. Well, nerve 12, the hypoglossal nerve. This is a nerve that controls the extrinsic muscles of the tongue. So if you stick your tongue out. So, everyone stick your tongue out, straight out. That's your Hypoglossal nerve in action and hopefully your tongue went straight, because if it didn't go straight, if it deviated to one side or the other, that could be a very important clinical sign and we'll spend some time talking about that one. Alright, I think we're ready now to look at the brain stem. So, I think you've seen the brain stem already. If, if you haven't you might want to, back up just a bit. And, view, my tour of the human brain, that I provided in some of the earlier neuranatomy tutorials. But, for our purposes here, I want you to focus on some of the key features of each of the 3 main divisions of the brain stem. So that, then, you'll know how these cranial nerves attach. So what we're looking at is a simplified illustration of the ventral surface of the brain stem. with some additional structures attached up above. So, we won't worry so much about what we see up in this region. that's actually part of the forebrain including structures like the basal ganglia and the thalamus. so we don't have the olfactory bulbs here so we're not going to see nerve one so we won't worry about that. I'll first just highlight this region, this junctional region, between the caudal diancephalon and the midbrain. So this region shown in this purple color is in fact the midbrain and we know that because of these broad stalks of white matter that we have on the ventral lateral aspect of the midbrain. That's called the cerebral peduncle. The space between the peduncle is called the interpeduncular fossa. And that's where we find. Cranial nerve three. So here's cranial nerve three, emerging from the inter peduncular fossa. Now, on the dorsal aspect of this brain stem right near the junction of the midbrain and the pons, which is just inferior to the midbrain, we see a little bit of the fourth nerve. Peeking out from the dorsal aspect of the brain stem. There it is on the other side, here. That's cranial nerve four, the trochlear nerve. What will be important to remember about the trochlear nerve is that it's the only motor nerve that exits on the dorsal side. Of the central nervous system all the other motor cranial nerves and the ventral roots of the spinal cord are all exiting on the ventral or anteiror aspect. Okay, well, just velow the midbrain, we have the ponds. The ponds is a large baldless like structure that is impossible to miss when you are looking at the ventral suface of the brain stem. Ponds is one of those great words in neuroanatomy that is worth knowing somehting about Pons means bridge, and the idea is that there is a long axis to the brain stem, and then the pon forms a bridge around this long axis to the brain stem. At roughly a 90 degree angle. So the pons contains these axons that are running. In the medial to lateral direction. And these axons, we'll learn more about them when we talk about the cerebellum. They're providing input to the cerebellum. So out here, we have the cerebellum. And these axons are terminating. Leaving the pons, terminating in the cerebellum, through a structure that we'll call the middle cerebellar peduncle. Well, I'm getting ahead of myself. For now I just want you to be able to recognize the pons, as this bit bridge across the longitudinal axis to the brain stem. All right. Well, as you can see from this illustration, once we get below the pons things get real busy real fast with respect to cranial nerves. So I want you to notice the junctional region between the pons and the medulla oblongata or just the medulla for short. So along this junction, We've got three pairs of cranial nerves and below that junctional region I want you to notice that the medulla has something of a longitudinal axis to it. There's sort of a seris of bulges and furrows in the longitudinal axis. So that defines some locations where I want you to look for certain cranial nerves. The medial of these two bulges is very important. We'll spend a lot of time talking about those structures. That's the medullary pyramid. That's the coritcal spinal tract that allow the cortex to control the m-, the actions of the spinal cord. Just to the lateral side of that medullary pyramid is a really lovely bulging structure. Which is called the olive. It sort of looks like an olive cut in the long dimension of, that fruit and then stuck to the side of the brain stem. That's the olive. So we're going to look for cranial nerves between the medullary pyramid. And the olive. And then we'll look for cranial nerves that are just lateral to the olive. Alright, well let's go ahead and look at a similar illustration of the ventral surface of the brain stem, now with all the nerves attached and labeled and color coded as to whether it is a purely sensory nerve, a purely Motor nerve, or a nerve that has both sensory axons and motor axons. So again, ignoring nerve one, which is in the anterior portion of the cranial floor, we have the optic nerve which comes together just in the Anterior part of the hypothalamic region, the base of the diancephelon. We see the nerves coming together just in front of a structure called the infundibulum or the stalk of the pituitary gland. We'll see that nicely in the labortoary. Well, also just behind this, this region, we have the mammillary bodies an obvious feature of the posterior floor of the hypothalamus. When you see the mammillary bodies, you know you're about to transition into the mid-brain. And that would be also indicated by the prominent cerebral peduncles and so now we know to look for nerves three on the ventral surface and nerve four emerging from the dorsel aspect of the mid brain. Well, The pons is what's next, going in the inferior direction. And plunging right through these transverse fibers of the pons is the trigerminal nerve. So that's an easy one to find, you'll know where to look for that. It's the only cranial nerve that seems to be merging from the ventral lateral aspects of that beautiful bridge across the long axis of the brain stem. Well that brings us to the junctional region between the pons and the medella. Where we find cranial nerves six, seven, and eight. And they are in medial telateral sequence with nerve six, the obducance nerve, close to the mid-line. And then, some distance out, we encounter nerves seven and eight. Which are actually rather close together. This illustration makes them look like they're sort of evenly spaced. But they're really not. Nerve seven is right on the medial side of nerve eight. With nerve eight being the most lateral. Now, if we eye the place where we find cranial nerve 8. We'll find nerves 9 and 10 just below cranial nerve 8. So the glossopharyngeal nerve is this one right here. And then just below it are a series of nerve rootlets, that comprise the rootlets of the vagus nerve, nerve number ten. And then roughly in the same position with respect to the lateral aspect of the lower part of the medulla, we find the spinal accessory nerve, nerve number eleven. And as I mentioned, there are rootlets that gather up from the upper cervical segments of the spinal cord. They form a nerve that then enters through the foramen magnum which is roughly about this position here. So there's our foramen magnum at the base of the skull. And this nerve enters then the cranial vault and it receives a small contribution for the medulla and together with nerves nine and ten. It exits the cranium via the jugular foramen so in that sense it's a real cranial nerve. And then lastly, the hypoglossal nerve. I mentioned the medial medullary pyramids right along the midline of the medulla. Here they are here. And then just to the lateral aspect of the. Medullary pyramids is this olivary structure, the inferior olive. And between the pyramid and the olive then that's where we find our hypoglossal nerve. So it's hard to see in real brains after they've been removed from the skull often these tiny rootlets will tear. But hopefully I'll be able to give you a view of the hypoglossal nerve roots in a brain specimen at some point during this course.
